The goal of this project is to expose the biochemical steps in the development of urolithiasis and kidney calcinosis in the young rat fed a low magnesium diet. An hypothesis is put forth suggesting that the initial crystal formation occurs within the vacuolar apparatus and that this matrix mass with its seed crystals is expelled into the tubular lumen by exocytosis. The hypothesis will be tested by isolation of the vacuolar bodies and measurement of their calcium phosphate binding, and isolation of the mineralized concretions and analysis of their organic core. A light and electron microscopic study will be conducted to complement the biochemical studies.